JP2002261843A - Differential signal transmission circuit - Google Patents

Abstract

PROBLEM TO BE SOLVED: To solve a problem that when the length of a transmission line is altered, the differential signal transmission circuit is remounted after a termination resistance is recalculated and extra labor and cost are required. SOLUTION: The differential signal transmission circuit comprises a constant voltage differential signal transmission system differential driver 5 and a differential receiver 6 wherein a differential signal from the differential driver 5 is transmitted on a transmission line 3 and received by the differential receiver 6 through a termination resistor 4. The differential signal transmission circuit further comprises means 7 for measuring the impedance of the transmission line 3.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a differential driver and a differential receiver of a constant voltage differential signal transmission type which are used in image reading apparatuses such as scanners and electrophotographic copiers and liquid crystal displays of computer systems. The present invention relates to a differential signal transmission circuit having the same.

[0002]

2. Description of the Related Art A high-speed differential signal transmission circuit converts a parallel image signal into a serial image signal by a parallel / serial conversion unit built in a differential driver, and outputs the serial image signal at a constant current. As a differential signal by the differential driver. This differential signal is transmitted by the transmission line and input to the receiver. At this time, the differential signal transmitted by the transmission line flows through the terminating resistor mounted on the receiver-side substrate to generate a potential difference between both ends of the terminating resistor. The receiver reads the potential difference by a differential receiver, restores the image to a parallel image signal by a serial / parallel converter, and outputs the parallel image signal as an output image signal.

[0003] JP-A-2000-181591 discloses that
LVDS (Low Voltage Differential Signalin)
g; constant voltage differential signal) Describes an interface signal.

[0004]

In the above-described differential signal transmission circuit, it is necessary to match the impedance of the transmission line with the impedance of the terminating resistor in order to receive a signal having an optimized amplitude on the receiver side. In the conventional design, the terminating resistor is optimized to match the impedance of the transmission line at the design stage, so if the line length of the transmission line changes or the like, the terminating resistor on the board is changed. It will be implemented again after recalculation. This is laborious and costly.

According to the present invention, even when the line length of the transmission line is changed, it is not necessary for the user to measure the impedance of the transmission line each time, and the information of the impedance of the transmission line can be taken out. A general-purpose transmission line of any length can be adopted, the impedance of the transmission line measured on the differential driver side can be read out on the differential receiver side, and the termination resistance can be changed to an optimal value. It is an object to provide a signal transmission circuit.

[0006]

According to a first aspect of the present invention, there is provided a differential driver and a differential receiver of a constant voltage differential signal transmission system, wherein a difference from the differential driver is provided. A differential signal transmission circuit for transmitting a dynamic signal via a transmission line and receiving the transmitted differential signal via the terminating resistor by the differential receiver, wherein the differential signal transmission circuit has a measuring means for measuring the impedance of the transmission line. .

According to a second aspect of the present invention, in the differential signal transmission circuit according to the first aspect, the transmission line is an FPC (Fle).
xible Printed Circuit) or a transmission line of a twisted pair type cable.

The invention according to claim 3 is the invention according to claim 1 or 2
3. A differential signal transmission line according to claim 1, wherein a measurement result of said measuring means is input to a signal processing unit at a preceding stage of said differential driver, and is transmitted from said differential driver to said differential receiver via said transmission line. The measurement result of the measuring means can be taken out on the differential receiver side.

According to a fourth aspect of the present invention, in the differential signal transmission circuit according to the third aspect, the measurement result of the measuring means transmitted to the differential receiver side is input, and the termination resistor is set based on the measurement result. It has a termination resistance adjustment circuit that adjusts to an optimum value.

[0010]

FIG. 1 shows a first embodiment of the present invention. The first embodiment is an LVDS (Low Voltage)
This is an embodiment of a differential signal transmission circuit of a Differential Signaling (constant voltage differential signal) transmission method, and has a built-in measuring means for measuring the impedance of a transmission line.
A cable 3 of an arbitrary length is connected as a transmission line between the connector 1 and the connector 2, and a terminating resistor 4 is connected between output terminals of the connector 2 to connect the connector 2 between output ends of the cable 3. The terminating resistor 4 is connected via the terminal.
Cable 3 is made of FPC (Flexible Printed Circuit)
t) or a transmission line of a twisted pair type cable.

When transmitting an image signal, the parallel image signal is converted into a serial image signal by a parallel / serial converter (not shown), and is converted to a differential signal by a constant current output differential driver 5 to the cable 3. Is output. This differential signal is transmitted by the cable 3 via the connector 1, flows to the terminating resistor 4 via the connector 2, and generates a potential difference at both ends of the terminating resistor 4. The differential receiver 6 receives the serial image signal by reading the potential difference,
A serial image signal from the differential receiver 6 is converted into a parallel signal by a serial / parallel converter (not shown) and output as an output image signal.

A line impedance measuring circuit 7 as measuring means for measuring the impedance of the cable 3 is connected to the input side of the connector 1 and operates when no image signal is transmitted to measure the impedance of the cable 3. If the resistance value of the terminating resistor 4 is determined in advance, it is possible to measure only the impedance of the cable 3 connected between the connectors 1 and 2 as a variation factor. Based on the measurement results of the line impedance measurement circuit 7, applications such as controlling the output of the differential driver 5 and matching the resistance value of the terminating resistor 4 to the impedance of the cable 3 can be performed.

According to the first embodiment, since the line impedance measuring circuit 7 as the measuring means for measuring the impedance of the cable 3 as the transmission line is provided, the case where the line length of the transmission line is changed or the like occurs. However, the user does not need to measure the impedance of the transmission line each time, and the information on the impedance of the transmission line can be extracted.

According to the first embodiment, since the transmission line 3 is a FPC (Flexible Printed Circuit) or twisted pair cable transmission line, the transmission line can be changed to an arbitrary one according to the application. ,
It is possible to use a general-purpose transmission line having an arbitrary length used in the current LVDS transmission.

FIG. 2 shows a second embodiment of the present invention. In the second embodiment, a signal processing unit 8 for performing signal processing such as converting a parallel image signal into a serial image signal is provided in a stage preceding the differential driver 5 in the first embodiment. The differential driver 9 includes the differential driver 8 and the differential driver 5. Further, a signal processing unit 10 for performing signal processing such as converting a serial image signal from the differential receiver 6 into a parallel image signal is provided at a subsequent stage of the differential receiver 6, and the signal processing unit 10 and the differential receiver 6 Thus, the receiver 11 is configured.

When transmitting an image signal, the parallel image signal is converted into a serial image signal by the signal processing section 8 and output to the differential driver 5, and the serial image signal from the differential receiver 6 is converted into a signal. The signal is converted into a parallel image signal by the processing unit 10 and output as an output image signal.

The line impedance measuring circuit 7 operates when no image signal is transmitted, measures the impedance of the cable 3, and outputs the measurement result to the signal processing section 8 as the impedance data S1 of the cable 3. The signal processing unit 8 receives the data S from the line impedance measurement circuit 7.
1 is output to the differential driver 5 as serial data,
The differential driver 5 outputs the data to the cable 3 as a differential signal. This differential signal is transmitted by the cable 3 via the connector 1, flows to the terminating resistor 4 via the connector 2, and generates a potential difference at both ends of the terminating resistor 4. The differential receiver 6 receives the serial data by reading the potential difference, and the serial data from the differential receiver 6 is converted into parallel data by the signal processing unit 10 and output as output data S2.

For example, when an image signal is transmitted from a personal computer to a liquid crystal display device using the differential signal transmission circuit of the present embodiment by using the cable 3 as a general-purpose interface cable, a line impedance measurement mode may be provided. By operating the line impedance measurement circuit 7, the impedance of the cable 3 can be displayed on the liquid crystal display device based on the output data S2 from the signal processing unit 10. This allows the user to know the impedance of the interface cable 3 arbitrarily selected and connected, and to change the terminating resistor 4 in accordance therewith so that the impedance of the interface cable 3 and the terminating resistor 4 can be matched. Become.

According to the second embodiment, the measurement result of the line impedance measuring circuit 7 as the measuring means is input to the signal processing section 8 at the preceding stage of the differential driver 5, and the cable as a transmission line is transmitted from the differential driver 5 to the cable. 3, the transmission result is transmitted to the differential receiver 6 side, and the measurement result of the line impedance measuring circuit 7 can be taken out at the differential receiver 6 side. Therefore, the impedance of the transmission line measured at the differential driver side is measured at the differential receiver side. It becomes possible to read.

FIG. 3 shows a third embodiment of the present invention. In the third embodiment, a variable resistor 12 is connected in parallel with the terminating resistor 4 in the second embodiment,
A terminating resistance adjusting circuit 13 for adjusting the variable resistor 12 according to the output data S2 is added. The terminating resistor 4 and the variable resistor 12 are substantially terminating resistors. In other words, the terminating resistor is constituted by a parallel circuit of the fixed resistor 4 and the variable resistor 12 so that the combined resistance value matches the impedance of the cable 3 in advance. I can decide.

The terminating resistance adjusting circuit 13 operates together with the line impedance measuring circuit 7 when the image signal is not transmitted, and changes the variable resistance 12 according to the output data S2 of the signal processing unit 10 so that the combined resistance value of the resistances 4 and 12 is changed. It is automatically adjusted to an optimum value that matches the impedance of the cable 3.

For example, an arbitrary interface cable is connected between the connectors 1 and 2 as the cable 3, and the impedance of the interface cable 3 is measured by the line impedance measuring circuit 7. The measurement results of the line impedance measurement circuit 7 are converted into data S1 as a signal processing unit 8, a differential driver 5, a connector 1, an interface cable 3, a connector 2, a terminating resistor 4 and a variable resistor 12, a differential receiver 6, and a signal processing unit 10. Output data S2 through
Is input to the termination resistance adjustment circuit 13, and the termination resistance adjustment circuit 13 sets the variable resistance 12 to the resistance
By adjusting the combined resistance value of 2 to match the impedance of the cable 3, the resistance values of the terminating resistors 4 and 12 always match the impedance of the interface cable 3 for an arbitrary interface cable 3. Therefore, it is possible to transmit a differential signal with an optimal amplitude waveform.

According to the third embodiment, the measurement result (the line impedance measurement circuit 7) transmitted to the differential receiver 6 is input so that the termination resistance becomes an optimum value based on the measurement result. Termination resistance adjustment circuit 13 to be adjusted
Therefore, the terminating resistors 4 and 12 can be changed to optimal values, and it is possible to always transmit a differential signal with an optimal amplitude waveform to an arbitrarily connected transmission line.

The variable resistor 12 is connected in series with the resistor 4, and the combined resistance value of the resistors 4 and
May be determined so as to match the impedance.

[0025]

As described above, according to the first aspect of the present invention, it is not necessary for the user to measure the impedance of the transmission line every time the line length of the transmission line is changed. Information on the impedance of the transmission line can be extracted. According to the invention according to claim 2, the transmission line can be changed to an arbitrary one according to the application.

According to the third aspect of the invention, the impedance of the transmission line measured on the differential driver side can be read on the differential receiver side. According to the fourth aspect of the present invention, it is possible to always transmit a differential signal with an optimum amplitude waveform to an arbitrarily connected transmission line.

[Brief description of the drawings]

FIG. 1 is a block diagram showing a first embodiment of the present invention.

FIG. 2 is a block diagram showing a second embodiment of the present invention.

FIG. 3 is a block diagram showing a third embodiment of the present invention.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1, 2 Connector 3 Transmission line 4 Termination resistance 5 Differential driver 6 Differential receiver 7 Line impedance measurement circuit 8, 10 Signal processing unit 12 Variable resistance 13 Termination resistance adjustment circuit

Claims (4)

[Claims] A differential driver and a differential receiver of a constant voltage differential signal transmission system, wherein a differential signal from the differential driver is transmitted through a transmission line, and the transmitted differential signal is terminated with a terminating resistor. A differential signal transmission circuit for receiving by the differential receiver through the differential signal transmission circuit, comprising a measuring unit for measuring the impedance of the transmission line. 2. The differential signal transmission circuit according to claim 1, wherein the transmission line is an FPC (Flexible Printed Cited Circuit).
rcuit) or a transmission line of a twisted pair type cable. 3. The differential signal transmission line according to claim 1, wherein a measurement result of said measuring means is input to a signal processing section in a preceding stage of said differential driver, and is transmitted from said differential driver through said transmission line. A differential signal transmission circuit configured to transmit the measurement result to the differential receiver side, and to be able to take out the measurement result of the measuring means on the differential receiver side. 4. The differential signal transmission circuit according to claim 3, wherein a measurement result of said measuring means transmitted to said differential receiver side is input so that said terminal resistance becomes an optimum value based on said measurement result. A differential signal transmission circuit comprising a termination resistance adjusting circuit for adjusting.